Tribasic Sodium Phosphate

Na3PO4 (anhydrous) 163.94
Trisodium phosphate, monohydrate     181.96
Phosphoric acid, trisodium salt, dodecahydrate;    
Trisodium phosphate, dodecahydrate     380.13 [10101-89-0].
Anhydrous     [7601-54-9].
DEFINITION
Tribasic Sodium Phosphate is anhydrous or contains one to twelve molecules of water of hydration. Na3PO4 (anhydrous) and Na3PO4·H2O (monohydrate) contain NLT 97.0% of Na3PO4, calculated on the ignited basis. Na3PO4·12H2O (dodecahydrate) contains NLT 92.0% of Na3PO4, calculated on the ignited basis.
IDENTIFICATION
•  A. Identification Tests—General, Sodium 191 and Phosphate 191: A solution (1 in 20) meets the requirements.
ASSAY
•  Procedure
Sample:  5.5 g of Tribasic Sodium Phosphate, on the anhydrous basis
Blank:  100.0 mL of 1 N hydrochloric acid, accurately measured
Titrimetric system 
Mode:  Residual titration
Titrant:  1 N sodium hydroxide VS
Endpoint detection:  Potentiometric
Analysis:  Transfer the Blank to a 400-mL beaker, and titrate with the Titrant to the endpoint at a pH of 7.0. Record as the volume consumed, and designate as A. Transfer the Sample to a 400-mL beaker, add 100.0 mL of 1 N hydrochloric acid, and stir until dissolved. Pass a stream of carbon dioxide-free air, in fine bubbles, through the solution for 30 min to expel carbon dioxide, covering the beaker loosely to prevent any loss by spraying. Wash the cover and sides of the beaker with a few mL of water.
Titrate the excess acid potentiometrically with the Titrant to the inflection point at a pH of 4. Record the buret reading, and designate as B. Protect the solution from carbon dioxide absorbed from the air, and continue the titration with 1 N sodium hydroxide VS to the inflection point at a pH of 8.8. Record the buret reading, and designate as C.
Calculate the amount of Titrant consumed by the Sample to the first inflection point, correcting for the Blank (V1 = A B) and the amount of Titrant consumed by the Sample between the two inflection points (V2 = C B). If V1 is equal to or greater than 2V2, calculate the amount of Na3PO4 in the portion of Sample taken:
D = V2 × N × F
V2== volume of Titrant consumed between the two inflection points (mL)
N== actual normality of the Titrant (mEq/mL)
F== equivalency factor, 163.9 mg/mEq
If V1 is less than 2V2, calculate the amount of Na3PO4 in the portion of Sample taken:
D = (V1 V2) × N × F
V1== volume of the Titrant consumed to the first inflection point, correcting for the Blank (mL)
N== actual normality of the Titrant (mEq/mL)
F== equivalency factor, 163.9 mg/mEq
Calculate the percentage of Na3PO4 on the ignited basis in the portion of Tribasic Sodium Phosphate taken:
Result = [10/(100 – L)] × (D/W)
L== percentage calculated in the test for Loss on Ignition 733
=
D== amount of Na3PO4 found (mg)
W== weight of the Sample (g)
Acceptance criteria:  NLT 97.0% of Na3PO4 on the ignited basis. Na3PO4·12H2O (dodecahydrate) contains NLT 92.0% of Na3PO4 on the ignited basis.
IMPURITIES
•  Loss on Ignition 733
Sample:  2 g
Analysis:  Dry the Sample at 110 for 5 h, and then ignite at 800 for 30 min.
Acceptance criteria:  The anhydrous form loses NMT 2.0% of its weight, the monohydrate loses 8.0%–11.0% of its weight, and the dodecahydrate loses 45.0%–57.0% of its weight.
•  Arsenic, Method I 211
Test preparation:  Dissolve a portion equivalent to 1.0 g of anhydrous tribasic sodium phosphate in 35 mL of water.
Analysis:  Proceed as directed in the chapter.
Acceptance criteria:  NMT 3 ppm
•  Heavy Metals, Method I 231
Test preparation:  Dissolve a portion equivalent to 2.0 g of anhydrous tribasic sodium phosphate in 25 mL of water.
Analysis:  Proceed as directed in the chapter.
Acceptance criteria:  10 ppm
SPECIFIC TESTS
•  Insoluble Substances
Sample solution:  Dissolve a portion equivalent to 10.0 g of anhydrous tribasic sodium phosphate in 100 mL of hot water.
Analysis:  Filter the Sample solution through a tared filtering crucible. [Note—Do not use glass. ] Wash the insoluble residue with hot water, and dry at 105 for 2 h.
Acceptance criteria:  The weight of the residue so obtained does not exceed 20 mg (0.2%).
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in tight containers. No storage requirements specified.
•  Labeling: Label it to indicate whether it is anhydrous, the monohydrate, or the dodecahydrate.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Robert H. Lafaver, M.S.
Scientific Liaison
1-301-816-8335
(EXC2010) Monographs - Excipients
USP35–NF30 Page 1957
Pharmacopeial Forum: Volume No. 32(2) Page 402